A Syrian air strike on a headquarters of a rebel brigade along the Turkish border killed at least five people, including children, and wounded dozens more on April 30, opposition activists said.

The attack targeted buildings belonging to the Ahrar al-Sham, a Salafist Islamist rebel unit fighting to topple Syrian President Bashar al-Assad, the activists said. A Turkish aid worker said the strike also hit a warehouse on the Syrian side of the border used by aid groups.

"The target appears to be Ahrar al-Sham but most of the fighting brigades have a presence at and around the crossing and it is impossible to get them without harming civilians," said Mohammad, an activist at the crossing, who gave only his first name.

"This is the closest air strike we have seen to the border. The crossing had been seen as a safe haven before," he said.

Another Syrian activist at Bab al-Hawa said people waiting to cross were among those hit. He added that at least 15 wounded were taken to hospital near the crossing on the Syrian side and among the dead were a one-and-a-half-year-old child and two teenage girls.

He said other casualties, many of them in critical condition, were taken to the Turkish towns of Reyhanli and Antakya.

Health officials in the Turkish border town of Reyhanli said the local hospital there, which frequently receives Syrian patients from just over the border, had taken "precautions" because of unconfirmed reports that some of the wounded may have come into contact with chemical weapons.

Some Syrian activists said some of the casualties were suffering breathing difficulties but said they did not know what type of munitions had been used in the attack.